1. Field of the Invention
The present invention relates to an electrophoretic display method capable of reversibly changing a display between the visually recognizable states by the action of an electric field or the like. The present invention also relates to an electrophoretic display liquid and display particles for use in the above-mentioned electrophoretic display method, an electrophoretic display medium for use with the above-mentioned electrophoretic display method, and further an electrophoretic display apparatus and reversible display material using the display medium.
2. Discussion of Background
The display apparatus using electrophoretic migration is constructed in such a configuration that two substrates, at least one substrate being made transparent, are disposed opposite via spacers to have a predetermined enclosed space therebetween. A display liquid is held in the enclosed space between the substrates to provide a display panel. The conventional display liquid comprises a dispersion medium and a pigment powder dispersed therein, the dispersion medium assuming a color different from that of the pigment powder. By the application of an electric field to the display panel, visual display can be observed through the transparent substrate serving as a display surface.
In the above-mentioned case, more specifically, the display liquid for use in the electrophoretic migration display method comprises a dispersion medium such as xylene or isoparaffin, a dispersed powder such as titanium dioxide, a dye for providing the dispersion medium with a different color from the color of the dispersed powder, a dispersant such as a surfactant, and an additive component such as a charge imparting agent.
By the application of an electric field to the above-mentioned display liquid, the dispersed powder in the liquid migrates to the transparent substrate side, so that a color of the dispersed particle can be recognized through the transparent substrate.
When the applied electric field is reversed, the dispersed powder migrates toward the other substrate, so that a color of the dispersion medium, which is colored with a dye, can be visually recognized through the transparent substrate.
The electrophoretic migration type display apparatus can provide a predetermined display by controlling the polarity of the applied electric field, as previously explained. Special attention has been paid to the electrophoretic migration type display apparatus because of the following advantages:
(1) The display apparatus is inexpensive since a display liquid for use in the apparatus is relatively easily available.
(2) The angle of field of the display apparatus is as wide as that of general printed matter.
(3) The consumption power can be reduced.
(4) The display apparatus has memory characteristics.
A display apparatus using electrophoretic migration is disclosed in Japanese Laid-Open Patent Application 1-86116, which is constructed in such a manner that a display liquid comprising a dispersed powder and a dispersion medium dyed with a color different from the color of the dispersed powder is enclosed in microcapsules, and the microcapsules are arranged between a pair of electrodes. This application is issued as Japanese Patent No. 2551783 in 1996. The electrophoretic display apparatus can be thus achieved by employing a simple structure.
In the display liquid for use in the conventional electrophoretic display apparatus, an inorganic pigment with a high refractive index, for example, titanium oxide powder, is dispersed in a dispersion medium in which a dye is dissolved for coloring the dispersion medium. However, when the color of the inorganic pigment powder is displayed, the color or the pigment powder is readily mixed with the color or the dispersion medium, thereby drastically lowering the contrast of display.
This problem is considered to result from adsorption of a dye for use in the display medium by the surfaces of the pigment particles, and permeation of the dye solution through the gap between the pigment particles. This problem is unavoidable for the electrophoretic display apparatus where a colored dispersion medium is employed for the display liquid.
The above-mentioned problem becomes noticeable when a white pigment is used as the dispersed powder. The degree of whiteness of a background portion in display, that is, a non-recorded portion, is considerably decreased, which may result in a fatal defect for the display medium.
To solve the above-mentioned problem, the following proposals are made: (a) to use a dye which is not readily adsorbed by the surfaces of pigment particles as a coloring agent for a dispersion medium (Philips Lab: Conference Record of 1980 Biennial Disp. Res. Conf.); (b) to decrease the concentration of a dye in a dispersion medium (Xerox PaloAlto: Proc. SID, Vol. 18, 3/4, 1977); and (c) to optimize the concentration of a dye, the concentration of a pigment, and the content of a surfactant (Matsushita: Proc. SID, Vol. 18, No. 3/4, 1977).
However, the above-mentioned proposals cannot produce sufficient effects in practice. To make matters worse, the density of a displayed image and the response speed are decreased.
As mentioned above, Japanese Patent No. 2551783 discloses a display method using display particles which are prepared by enclosing an electrophoretic display liquid in microcapsules. One of the advantages obtained by this electrophoretic display method is that the problem of uneven display as caused by non-uniform distribution of electrophoretic particles can be inhabited. However, even when the above-mentioned display method is adopted, the pigment powder tends to be colored by a dye solution, thereby lowering the display contrast because each microcapsule contains therein a dye solution and a dispersion of a pigment powder.
An electrophoretic display system using no dye solution is proposed in order to eliminate the above-mentioned shortcoming resulting from the dispersion medium colored with a dye.
For instance, Japanese Laid-Open Patent Application No. 62-269124 discloses an electrophoretic migration type display element constructed in such a manner that opposite electrodes, at least one of which is transparent, are superimposed via spacers to provide a cell therebetween, and a display liquid is enclosed in the cell. The display liquid comprises a colorless dispersion medium which has high electrical insulating properties and low viscosity, and at least two kinds of electrophoretic migration powders dispersed in the dispersion medium, those electrophoretic powders being different in color and polarity for electrophoretic migration.
However, there is a risk that the above-mentioned two kinds of electrophoretic migration powders with different colors tend to aggregate by electrical attraction because the two kinds of powders are charged to opposite polarities. Consequently, color mixing occurs, and it becomes difficult to achieve a display with high contrast.
According to Japanese Laid-Open Patent Application 63-50886, at least two kinds of electrophoretic migration powders are dispersed in a colorless dispersion medium which has high electrical insulating properties and low viscosity, thereby preparing a display liquid. The display liquid is held in the cell in the same manner as in the above, so that an electrophoretic migration type display element is provided. In this case, however, the two electrophoretic migration powders show the same polarity, but are different in color tone and electrophoretic velocity.
In this case, image display is attained on the ground of the difference in migration velocity of the electrophoretic migration powders with different colors. Those powders have inclinations to migrate in the same direction, so that it is impossible to simultaneously display two different colors on the identical display surface. This image display method is therefore considered to be impractical.
Further, WO98/03896 discloses an embodiment where the same dispersion system as in the aforementioned Japanese Laid-Open Patent Application No. 62-269124 is enclosed in microcapsules. In this case, the two kinds of electrophoretic migration powders are electrically attracted to each other, so that the powders tend to aggregate. Namely, the color mixing problem occurs in each microcapsule. The preparation of the microcapsules is impractical in this case.
To prevent such aggregation of the two kinds of electrophoretic powders which are different in color and electrical charge for use in the display liquid, the addition of a charge control agent to the display liquid and surface treatment of the powders are proposed in Published Japanese Translation (No. 8-510790) of PCT international publication for patent application. However, the electrophoretic powders with opposite electric charges to migrate in opposite directions cannot be completely prevented from being aggregated. High contrast has not yet been obtained in such a display element of electrophoretic type.
An ink composition for image display is disclosed in Japanese Laid-Open Patent Application 10-149117, which comprises a concealing white powder comprising a resin and a white pigment, a coloring powder for image display, and a solvent.
In the above application, the white powder is a composite material prepared by subjecting a mixture of a white pigment and a resin to polymerization and aggregation after kneading, pulverizing, and dispersing steps. This type of white powder is employed in the display liquid because the specific gravity of the white powder is different from that of the dispersion medium colored with a dye. The white powder is not provided with a function of preventing the two kinds of powders from being aggregated when the white powder is used in combination with another coloring powder, that is, a magnetic powder or a mixture comprising the magnetic powder. The result is that there also occurs the above-mentioned problem of color mixing, and the contrast is lowered in the obtained display.
Accordingly, a first object of the present invention is to provide an electrophoretic display method using a display dispersion system comprising a dispersion medium and at least two kinds of electrophoretic migration powders with different colors, capable of achieving a display with high contrast.
A second object of the present invention is to provide an electrophoretic display liquid for use with the above-mentioned electrophoretic display method.
A third object of the present invention is to provide electrophoretic particles for use with the above-mentioned electrophoretic display method.
A fourth object of the present invention is to provide an electrophoretic display medium using the above-mentioned electrophoretic display liquid and display particles.
A fifth object of the present invention is to provide a display apparatus using the above-mentioned electrophoretic display medium.
A sixth object of the present invention is to provide a reversible display material using the above-mentioned electrophoretic display medium.
The previously mentioned first object of the present invention can be achieved by an electrophoretic display method, using a display dispersion system, the display dispersion system comprising a dispersion medium, at least one electrophoretically movable coloring material (A) comprising coloring particles, and at least one electrophoretically immovable coloring material (B) comprising coloring particles, each of the coloring materials (A) and (B) having a different color tone and being dispersed in the dispersion medium, comprising the step of selectively causing the coloring material (A) to electrophoretically migrate through the dispersion medium in a predetermined direction in response to an electric field externally applied thereto, while causing the electrophoretically immovable coloring material (B) to stay in the dispersion medium.
The second object of the present invention can be achieved by an electrophoretic display liquid for use with an electrophoretic display method, comprising a dispersion medium, at least one electrophoretically movable coloring material (A) comprising coloring particles, and at least one electrophoretically immovable coloring material (B) comprising coloring particles, each of the coloring materials (A) and (B) having a different color tone and being dispersed in the dispersion medium, the coloring material (A) being selectively caused to electrophoretically migrate through the dispersion medium in a predetermined direction in response to an electric field externally applied thereto, while the electrophoretically immovable coloring material (B) being caused to stay in the dispersion medium.
The third object of the present invention can be achieved by electrophoretic display particles comprising microcapsules which contain the above-mentioned electrophoretic display liquid therein.
The fourth object of the present invention can be achieved by an electrophoretic display medium comprising (i) a pair of substrates, at least one of the substrates being transparent and provided with an electrode, the substrates being disposed to face each other via spacers or not, to have an enclosed space therebetween in such a configuration that the electrode surface of one substrate faces the other substrate, and (ii) the above-mentioned electrophoretic display liquid held in the enclosed space between the substrates.
The electrophoretic display medium may be prepared in such a configuration that the enclosed space is partitioned by a matrix material which holds the display liquid therein.
Further, in the electrophoretic display medium, the above-mentioned electrophoretic display particles may be held in the enclosed space between the substrates. In this case, the enclosed space may be partitioned by a matrix material which holds the electrophoretic display particles therein.
Furthermore, the fourth object can also be achieved by an electrophoretic display medium comprising (i) a substrate having a surface provided with an electrode, and (ii) a recording layer provided on the electrode surface of the substrate, the recording layer comprising a matrix material and the above-mentioned electrophoretic display liquid or electrophoretic display particles.
The fifth object of the present invention can be achieved by an electrophoretic display apparatus comprising the above-mentioned electrophoretic display medium and a recording unit capable of displaying a visible information on the display medium, the recording unit coming in contact with the display medium at least in recording operation. The recording unit comprises an electrode array which can apply an electric field to the display medium according to image signals and shift the relative position on the same plane with respect to the display medium.
The recording unit for use in the electrophoretic display apparatus may comprise an ion beam array which can impart electric charges to the display medium according to image signals and shift the relative position on the same plane with respect to the display medium.
In addition, the recording unit for use in the electrophoretic display apparatus may comprise a plurality of signal electrodes and a plurality of scanning electrodes, each of the intersecting points of the signal electrodes and the scanning electrodes having a switching element capable of applying an electric field to the display medium according to image signals.
The sixth object of the present invention can be achieved by a reversible display material comprising the above-mentioned electrophoretic display medium.